IT EVOLVES CARBONIC ACID. 29 



Woody fibre in a state of decay is the substance called 

 humus.* 



The property of woody fibre to convert surrounding oxygen 

 gas into carbonic acid diminishes in proportion as its decay 

 advances, and at last a certain quantity of a brown coaly-looking 

 substance remains, in which this property is entirely wanting. 

 This substance is called mould ; it is the product of the complete 

 decay of woody fibre. Mould constitutes the principal part of 

 all the strata of brown coal and peat. By contact with alkalies, 

 such as lime or ammonia, a further decay of mould is occa- 

 sioned. 



Humus acts in the same manner in a soil permeable to air as 

 in the air itself; it is a continued source of carbonic acid, which 

 it emits very slowly. An atmosphere of carbonic acid, formed 

 at the expense of the oxygen of the air, surrounds every particle 

 of decaying humus. The cultivation of land, by tilling and 

 loosening the soil, causes a free and unobstructed access of air. 

 An atmosphere of carbonic acid is therefore contained in every 

 fertile soil, and is the first and most important food for the young 

 plants growing upon it. 



In spring, when those organs of plants are absent which nature 

 has appointed for the assumption of nourishment from the atmo- 

 sphere, the component substances of the seeds are exclusively 

 employed in the formation of the roots. Eaeli new radicle fibril 

 acquired by a plant may be regarded as constituting at the same 

 time a mouth, a lung, and a stomach. The roots perform the 

 functions of the leaves from the first moment of their formation : 

 they extract from the soil their proper nutriment, namely, the 

 carbonic acid generated by the humus. 



By loosening the soil surrounding young plants, we favor the 

 access of air, and the formation of carbonic acid ; and, on the 

 other hand, the quantity of their food is diminished by every 

 difficulty which opposes the renewal of air. A plant itself effects 

 this change of air at a certain period of its growth. The car- 

 bonic acid, which protects the undecayed humus from further 



* The humic acid of chemists is a product of the decomposition of 

 humus by alkalies ; it does not exist in the humus of vegetable physiolo- 

 gists. 



